1. Field of the Invention
The present invention relates to a laser machining apparatus and particularly to correction of a laser output of a laser oscillator.
2. Description of Related Art
In machining with a laser machining apparatus, in order to keep machining performance constant, it is necessary to control the output value, or output power of a laser oscillator accurately. In the laser machining apparatus, the angle of a mirror is changed to change the path of a laser beam. Due to this change of the mirror angle, and other reasons such as optical components such as mirrors becoming soiled and high-frequency power source loss, the output power of the laser oscillator changes with age.
As a way to compensate for this secular change, there is known an invention in which relation between command values supplied to a laser oscillator and actual output values is obtained in advance and stored as a table, and in which a command value for obtaining a required laser output value is obtained on the basis of the table and supplied (see JP 3-62515B).
In the invention disclosed in this publication, it is necessary to change the table every predetermined period in view of the secular change, and remaking the table involves difficulties. Thus, another easy laser output correction method has been developed. In this method, when a laser oscillator is started up, the laser output is measured with an output power measuring device after the laser output stabilizes. Then, a correction coefficient is obtained from the measured laser output value and a command value. From this time onwards, a command value multiplied by this correction coefficient is supplied as a corrected command value to perform machining (see JP 2804027B).
Apart from the secular change of components of the laser machining apparatus, the temperature at which the laser oscillator operates changes the output power of the laser oscillator. Hence, also this temperature at which the laser oscillator operates needs to be taken into consideration.
In the above-mentioned invention disclosed in JP 2804027B, a laser output correction value can be obtained easily by measuring the laser output when the laser oscillator is started up. To the laser oscillator, coolant is supplied under water temperature control so that the laser oscillator can reach a steady state quickly. However, when the laser oscillator is being started up, also the water temperature control is being started up, so that the coolant is being supplied under imperfect water temperature control. It takes more time until the water temperature of the coolant comes to be kept at a desired value than until the laser output stabilizes. Hence, the laser output is measured while the water temperature is still changing, and the correction coefficient is obtained on the basis of the laser output thus measured.
Even when the laser output of the started-up laser oscillator is measured after the laser output stabilizes and the correction coefficient is obtained on the basis of the obtained laser output value, it only means that the correction coefficient is obtained in view of the secular change of the output power. It does not mean that the correction coefficient is obtained in view of the influence of temperature. Hence there is a problem that when the laser oscillator reaches a steady state where the water temperature is at a predetermined temperature, the laser output value changes, so that an accurate laser output value cannot be obtained.